Greywater

Author: Andre Baas (AKWSOLV, South Africa) & Erwin Nolde (NOLDE & PARTNER, Berlin, Germany)

( Article Type: Explanation )

Although there is no absolute consensus on the definition of greywater, it is widely accepted that greywater is defined as the wastewater from bath tubs, showers, washbasins and washing machines. Whilst Greywater may also include the more polluted wastewater from kitchen sinks, it does not include ‘blackwater’ - which is the wastewater from toilets that contains urine and/or faeces or ‘industrial effluent’ which is waste water from industrial operations or manufacturing processes – and typically contains problematic chemicals.

Greywater Composition
 The easily degradable organic substances originate mainly from detergents, skin grease, hair and skin and dandruff particles. Due to its ready biodegradability, if greywater is not treated soon, decomposition processes with the sulfates will take place leading to the development of unpleasant odours. The organic substances are measured by means of the parameters BOD (biological oxygen demand) or COD (chemical oxygen demand). The content of the organic substances is dependent on the origin of the collected greywater, and apart from the organic load, also contains inorganic nutrients such as nitrates and phosphates. Greywater from showers and bath tubs is the least polluted wastewater. When greywater from washing machines is used, a considerably higher concentration of substances in the greywater is expected and as a consequence, a higher treatment expenditure. The use of kitchen wastewater (sink, dishwasher) will further increase the load.

Greywater recycling
Greywater may be reused for other (non-potable) purposes, but not without purification and treatment. The least polluted flows of the available wastewater (bath, shower and bathroom hand-basins) is especially suitable for greywater recycling. Therefore the best candidates for greywater recycling include residences, apartment buildings, hotels, lodges and gyms.
A greywater recycling plant collects greywater and treats it to become ‘process water’ of sufficient quality to become usable for some other specific purposes. Process water is defined as “water with different quality characteristics’ that is suitable for application in commercial, industrial, agricultural or similar purposes”. Process water from recycled greywater is suitable for residential and commercial applications such as for toilet flushing, irrigation, for cleaning purposes or for laundry. The figure below provides a typical representation of the water lifecycle in a typical residence and is expressed as litres of water per person per day. This diagram below provides some insight into the source and application of fresh and wastewater and demonstrates the potential for greywater recycling.